Coal is primarily employed for conversion into electrical and thermal energy. A major disadvantage of coal which is mined in the United States is the high sulfur content, i.e., a sulfur content which ranges up to about 5 weight percent.
The annual rate of coal production in the United States has reached about 600 million metric tons on the basis of 1975 estimates. Approximately three-fourths of the coal production is burned directly for the generation of electricity and steam power, for comfort heating, for metallurgical processes and for kiln firing of ceramics.
A one million kilowatt plant burns about 8500 tons per day of coal. This corresponds to a stack emission of sulfur oxides equivalent to about 1000 tons of sulfuric acid per day, for combustion of coal fuel having a weight content of 3.5 percent sulfur.
The sulfur in coal occurs in three forms: (1) pyritic sulfur in the form of pyrite or marcasite, (2) organic sulfur, and (3) sulfate sulfur. The primary sulfur contaminants are of the first two forms.
Increased exploitation of coal as fuel is restricted because of the pollution problems inherent in plant scale combustion of coal for its energy value.
Continuous research and development effort is in progress for the study of coal pollution problems and for provision of improved pollution abatement technology.
Conventional systems for desulfurization of stack effluent gases are effective for pollution control, but such systems are too capital-intensive and impractical for general application.
Other developments for control of sulfur emission from coal burning furnaces involve the desulfurization of coal prior to combustion.
The use of manganese oxide to desulfurize coal and coal products has long been known in the art. These prior processes may be characterized as high-temperature volatilization processes. U.S. Pat. No. 28,543 discloses a process for the removal of sulfur after the coking process, wherein a mixture of sodium chloride, manganese peroxide, resin, and water is applied to the red-hot coke, and sulfur is oxidized and released from the coke mass in gaseous form. Other similar processes are disclosed in U.S. Pat. Nos. 90,677; 936,211; 3,348,932; and 3,635,695.
The use of oxidative solubilization processes to remove sulfur from coal is a relatively new concept. Even though the solubilization of pyrites by various oxidizing agents, including nitric acid, hydrogen peroxide, hypochlorite, ferric and cupric ions, has long been known, the application of these reactions to the removal of pyrite from coal is a recent development.
U.S. Pat. No. 3,768,988 describes a process which employs aqueous ferric sulfate or chloride to oxidize pyritic sulfur to coal to elemental sulfur: EQU 2Fe.sup.+3 + F.sub.2 S.sub.2 /coal .fwdarw. 3Fe.sup.+2 + 2S/coal
The elemental sulfur thus formed is removed by solvent extraction of the coal or by heat treatment of the coal to volatilize the elemental sulfur.
U.S. Pat. No. 3,824,084 discloses a method for production of low sulfur coal by treatment of high sulfur coal with water and air at elevated temperature and pressure to convert pyritic sulfur to water-soluble ferrous and ferric sulfate.
U.S. Pat. No. 3,909,211 describes a coal desulfurization process which involvee heat treatment of the coal in the presence of NO.sub.2, wherein sulfur in the coal is selectively oxidized into compounds which are more readily separable from the treated coal.
U.S. Pat. No. 3,909,213 discloses a coal desulfurization process which comprises digesting coal and a Group IA or IIA metal oxide in the presence of a fused metal chloride salt medium capable of dissolving sulfur-containing compounds in the coal. The metal sulfides produced are converted to metal chlorides and hydrogen sulfide by treatment with anhydrous hydrogen chloride.
It is also possible to remove pyritic sulfur from coal by froth flotation as reported in BuMines TPR 51 (1972) publication entitled "Flotation Of Pyrite From Coal". Froth flotation has the disadvantage of poor selective concentration of coal from its sulfur and ash components.
A DuPont gravity concentration minerals separation process is described in U.S. Pat. Nos. 2,150,899; 2,150,917; 2,150,918; 2,150,946; 2,150,947; and 2,151,578. The DuPont process, employing chloroethane parting liquid, was installed on a pilot-plant scale in the Pennsylvania anthracite fields in 1936. The raw feed with the fines screened out was sprayed with a solution of tannic acid or starch to prevent loss of the parting liquid by surface filming. The process was generally applicable for gross sink-and-float separation of ash and refuse from No. 1 buckwheat and larger size coal stock, and for salvaging fuel values from refuse feeds.
Coal desulfurization processes involving chemical means are in general relatively complicated and not economically attractive. As a simple solution to coal combustion pollution problems, coal having an acceptably low sulfur content has been the premium fuel of choice for energy conversion needs. Many pollution control districts prohibit combustion of coal having more than 1.0 percent sulfur content. This has disqualified the use of many United States coals, 90 percent of which average about 2.5 percent by weight of sulfur. There remains a need for improved coal desulfurization technology for converting high sulfur coal into a solid carbonaceous fuel which satisfies increasingly stringent pollution control regulations.
Accordingly, it is a main object of the present invention to provide an efficient and economical process for desulfurizing solid carbonaceous fossil fuels which have a sulfur content above 1.0 weight percent.
It is another object of the present invention to provide a process for reducing the sulfur content of coal without chemical conversion of the sulfur into elemental or other forms of sulfur.
It is another object of this invention to provide a process for separating pyritic sulfur and ash from finely divided coal in a liquid system without floc formation of the fine coal particles.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.